It needs to puke!

Poly can have an air entrapment when filling with coolant. I learned this trick 32 years ago: Remove thermostat, drill a 1/8"-3/16" hole in the flat plate on thermostat, roughly centered from middle section & outer rim.

Replace, and when refilling with coolant let it fill, wait a bit and watch it drop! Add more, repeat, may take over 1/2 hour to fully fill system.

There is no collateral problem with this, entirely safe, works on many vehicles. I now never install a thermy in anything without doing this, because all of the big 3 & others have some designs that get an air bubble when filling. This also speeds up thermostat opening in hot weather, more flow around element. There's one other advantage to it but I can't remember what it is.